In order to avoid contaminating the heat-seal surface of rigid and semi-rigid container bodies to be closed by a heat-seal diaphragm it is known to leave a "headspace" by which the surface level of the product falls short of the heat-seal surface.
A web of flexible material is then heat-sealed to the heat-seal surface to form a generally plane diaphragm closure, after which the diaphragm is severed around the container to separate it from the parent web material.
Because of the headspace which has been provided, such prior processes have left substantial residual air trapped within the container between the diaphragm and the product. This air has caused spoilage of oxygen-sensitive products and has hindered the exploitation of sterilizable containers closed by a heat-sealed diaphragm because of the difficulty of retorting the containers with a sufficient accuracy of pressure control to ensure that the heat-seals are not ruptured or the containers otherwise deformed or damaged, by the expansion or contraction of the included air during heating and cooling. Substitution of an inert gas in the headspace has relieved the problem of oxygen spoilage but not the heat-sterilisation problem.